Non-human primates are frequently used to test the efficacy and safety of new mucosal vaccines, i.e., vaccines that are most efficaciously applied directly to a mucosal membrane of a subject, but for various reasons, suitable primates that may have to be sacrificed at the end of a set of vaccine applications are very difficult to obtain and very expensive when available. Additionally, there is an increasingly strong public sentiment against the use of primates as test subjects. Because of such difficulties and expense, it is generally quite difficult to obtain statistically significant results to convincingly demonstrate the safety of a novel vaccine through the use of primates. Primate studies also tend to limit the flexibility of the investigations, e.g., because investigators are generally hesitant to modify or improve an imperfect vaccine for fear of having insufficient test animals to complete a credible study. In short, therefore, the lack of a suitable animal model for adequate testing of mucosal immunogens has resulted in this country in relatively limited testing or refinement of numerous candidate vaccines prior to clinical trials.
The rabbit appears to be an excellent test animal for such studies, not only because it is relatively small and easy to handle, relatively inexpensive to breed and hence commercially inexpensive to obtain, but also because it has a comparatively large mucosal immune system, a sensitive lower gastrointestinal tract and because rabbits have been used for mucosal immunization with satisfactory results and are therefore generally deemed by investigators to be a suitable animal model.
There are two broad categories of vaccine: those that are parentally administered by injection into muscle tissue and those which require mucosal administration, e.g., polio vaccine on a sugar cube. Parental vaccines, generally designed to stimulate systemic immune responses, are routinely tested in lower vertebrates such as mice, rats or rabbits and, only later, once the best candidates among the tested antigens are determined, are the vaccines tested in non-human primates and subsequently in humans. During the parental vaccine screening process in rodents or rabbits, systemic immune responses are measured and both local or systemic pathologic changes are noted.
The procedures suitable for testing of parental vaccine often are not followed in evaluating putative mucosal vaccines because mucosal immune responses are difficult to measure in small animals and because the number of experimental variables, e.g., the route of administration, the use of adjutants and the amount or form of the antigen, demand relatively large numbers of animals. As noted above, the rabbit is a suitable test animal for many reasons and the problem therefore resolves to one of obtaining safe, controlled and long term access to a suitably large portion of the animals' mucosal tissue for delivery thereto of liquid test materials.
Many studies have been performed with delivery of liquid materials to the intestinal mucosal tissues of rabbits by using a Thiry-Vella (T-V) loop system. This T-V system involves the surgical isolation and exteriorization of a section of the gut of the test animal. There are some significant disadvantages associated with use of the T-V system For example, data obtained through use of a T-V loop system has a very broad standard array, a consequence that is believed to be related to the modified physiological function of the intestine, e.g., Paneth cell hyperplasia, colonization with undesirable bacteria, and post-surgical intestinal inflammation. Furthermore, the so-called "T-V loop" is not truly a functional intestinal loop and, therefore, is not a very good model of a functioning physiologic system. Finally, mucus build-up within a T-V loop may restrict antigenic presentation to the lymphoid tissue, and one significant consequence of this is that there is a practical limitation in the usefulness of such T-V loops, usually less than 15 days For many sensitive studies, and especially to obtain a solid data base to justify the much more expensive follow-up studies needed in primates and human subjects, it is required that the test animal be available with a truly functional physiologic system to receive the liquid test materials for periods in excess of 15 days.
There is, therefore, a real need for a device and a method that will enable practitioners and researchers to deliver liquid test materials over a long period of time directly to a relatively large mucosal lymphoid tissue, e.g., in the lower gastrointestinal tract of the test animal, without undue expense of time or energy.